The present invention relates to a process for converting UF.sub.6 gas into UO.sub.2 powder which is suitable for manufacturing a reactor fuel owing to its high activity, low remaining fluorine amount and excellent fluidity.
As a process for converting UF.sub.6 gas into UO.sub.2 powder for a reactor fuel, there have been conventionally two processes, that is, a wet process and a dry process. The wet process is a process in which an uranyl ion containing solution is obtained by hydrolyzing UF.sub.6 in gas-liquid reaction, then the solution is added with reagent to be precipitated, and the precipitate is filtered, dried roasted, reduced to be UO.sub.2 powder. The UO.sub.2 powder obtained by the wet process is high in activity and low in remaining fluorine amount, but it is defective in that there are many steps which are complex and the generated volume of waste liquid is large. Especially, large load in the filtering step, low filterbility of the precipitate and uranium loss into the filtrate have been already pointed out.
On the other hand, in the case of the dry process, there is a process using a rotary kiln, a process using a fluid bed reaction apparatus and a process using a flame combustion reaction apparatus. Of these processes, the process using a fluid bed reaction apparatus forms UO.sub.2 powder as a product which has a very excellent fluidity, thus making the handling of UO.sub.2 powder in following steps very easy, as compared with the other processes. The UO.sub.2 powders obtained by almost all the conventional processes are bad in fluidity, so their handlings in the following steps are not easy.
As above described, in the process using a fluid bed reaction apparatus, there is obtained UO.sub.2 powder having an excellent fluidity thus making handling of the UO.sub.2 powder in the following steps very easy, but the activity of the UO.sub.2 powder becomes smaller and the remaining fluorine amount thereof becomes larger, as compared with these of the other processes. This lowering of the activity of the UO.sub.2 powder is due to formation of UO.sub.2 F.sub.2 by gas phase reaction of UF.sub.6 gas with steam as shown in the following equation (1) and formation of UF.sub.4 by conversion into UO.sub.2 of UO.sub.2 F.sub.2 with hydrogen gas as shown in the following equation (2).
In the conventional dry process, especially in the process using a fluid bed reaction apparatus, the conversion of UF.sub.6 gas into UO.sub.2 powder is following two stage reactions. EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (1) EQU UO.sub.2 F.sub.2 +H.sub.2.fwdarw.UO.sub.2 +2HF (2)
In this process, a reverse reaction of equation (2) is apt to form UF.sub.4. EQU UO.sub.2 +4HF.fwdarw.UF.sub.4 +2H.sub.2 O (3)
UF.sub.4 is a substance which is apt to sinter at relatively low temperature (about 1000.degree. C.) and it begins to sinter at the operating temperature of equation (2) to hinder a deflorinating reaction which is important for lowering a remaining fluorine amount of UO.sub.2 powder as a product. Therefore, it was formerely required to add in excess of steam in equation (2) to make equation (3) not occur. As a result, the operation became more complex and at the same time the excess added steam increased the amount of waste liquid. Further, as a long time was required for deflourinating the UO.sub.2 powder as a product, it was exposed to the high temperature for a long time. Consequently, activity of the UO.sub.2 powder is apt to be largely reduced.
Further, another defect of the case in which the fluid bed reaction apparatus is used relates to the operational stability of the fluid bed. Namely, the UO.sub.2 F.sub.2 particles form the fluid bed, but UF.sub.6 gas blown into the fluid bed reacts with steam as a fluidizing gas introduced through the bottom of the fluid bed to form UO.sub.2 F.sub.2 which sticks on the surface of UO.sub.2 F.sub.2 particles already existing. As the result, the thus sticky UO.sub.2 F.sub.2 causes growth of the UO.sub.2 F.sub.2 particles. On the other hand, a part of the UO.sub.2 F.sub.2 particles are pulverized by abrasion owing to collison with other UO.sub.2 F.sub.2 particles. The mean particle size of the UO.sub.2 F.sub.2 particle is determined by these balances, but in the conventional fluid bed reaction apparatus, the resultant UO.sub.2 F.sub.2 particles are apt to grow very large. Therefore, it was required to supply new UO.sub.2 F.sub.2 powder into the fluid bed to maintain a stable operation of the fluid bed. Consequently, the apparatus system became complex and the operation was complicated.